1. Field of the Invention
The present invention relates generally to temperature sensing. More particularly, the present disclosure relates to systems for calibrating temperature profiles in, for example, a distributed line system.
2. Description of Related Art
Optical fibers have been used for optical communication for decades. Recently, optical fiber sensing technologies have grown rapidly due to fiber advantages, which conventional electrical sensors do not have. The advantages of fiber include the ability to handle much higher bandwidth and inherently safe operation (no generation of electric sparks). Also optical fiber is inherently immune to EMI (ElectroMagnetic Interference), and it does not radiate EMI. A prominent feature of the fiber is the capability of true distributed parameter measurement. Utilizing this technology, temperature and strain profiles along significant distances can be monitored over extended lengths. Many temperature data points can be processed along a considerable length, over tens of kilometers. The resultant distributed measurement is equivalent to numerous conventional point temperature sensors which would require more deployment equipment and a higher operational costs.
When an optical fiber is excited with a laser light with a center wavelength λ, most of the light may be transmitted but small portions of incident light λ are scattered backward and forward along the fiber. Scattered light is categorized into three bands: Rayleigh, Raman, and Brillouin scatterings. For the measurement of distributed temperatures, a few components may be used such as a Rayleigh scattering, which is the same as an excitation wavelength λ, and a Stokes and anti-Stokes components which are longer and shorter than λ, respectively. These three components may be separated by optical filters and received by the photo detectors to convert the light to electrical signals. The ratio of temperature sensitive anti-Stokes intensity to temperature insensitive Rayleigh or Stokes intensities may be used for temperature measurement.
To obtain a local temperature profile along a distance, two methods—time domain approach and frequency domain approach have—been applied conventionally. The time domain method uses a pulsed light source and the position of the temperature is identified by the calculation of the pulse round trip time to the distance under test. The frequency method uses a modulated laser source and the position can be calculated by applying the inverse Fourier transformation of a sensing fiber's transfer function or frequency response.
U.S. Pat. No. 5,113,277, which is incorporated by reference, discloses a Fiber Optic DTS (Distributed Temperature Sensing) system, which involves a pulsed light source and a temperature measurement was made by the ratio between Stokes and anti-Stokes intensities at each measured distance determined from the roundtrip time of the pulse. U.S. Pat. No. 7,057,714, which is incorporated by reference, discloses a stepped modulation method to sweep the frequency of the laser source. The time domain profiles of Stokes and anti-Stokes attenuations are obtained by applying the inverse Fourier transformation of amplitude and phase responses of each modulating frequency component. The time domain method is simpler than frequency domain analysis but it requires a costly pulsed light source and higher data acquisition components but has a lower signal to noise characteristics.
The temperature profile along the sensing fiber in DTS is obtained by the ratio of the temperature insensitive Stokes to temperature sensitive anti-Stokes backscattered intensities from a deployed sensing fiber as described above. But both scattering intensities are also dependent on, for example, mechanical and chemical perturbations such as micro bends, tensions, compressions and chemical ingressions such as hydrogen gas, which are common in an oil field environment under high temperatures and high pressures. This kind of ambiguity, i.e., whether the scattering intensities are made by pure local temperature effect or by other effects mentioned above particularly in an anti-Stokes profile, usually introduces some errors in the temperature calculation and needs to be corrected to generate more accurate temperature measurements. This ambiguity may be corrected with the aid of conventional optical reflectometry methods, in which the backscattered light provides a measure of wavelength dependant attenuations. In order to implement this idea to the DTS system, an extra incident source with the same wavelengths and similar line width of anti-Stokes or Stokes bands is required. Commercial availability and/or the cost have been major obstacles for a practical implementation of this correction technique.
The referenced shortcomings above are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques for temperature profiling; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.